Inhibiting corrosion of ferrous metals



United States Patent INHIBITING CORROSION 0F FERROUS METALS John E. Malian, Bartlesville, Okla and Robert A. Stahl, Kansas City, Kans., assignors to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application June 23, 1952, Serial No. 295,118

Claims. c1. 23-151 This invention relates to methods of reducing corrosion of ferrous metals by dilute aqueous solution of nitric acid. In one of its aspects the present invention relates to a composition of matter of reduced corrosiveness with respect to ferrous metals.

Dilute aqueous nitric acid solutions are used extensively, in commercial operations such as in pickling baths, bleaching operations, and the purification and preparation of metals and their compounds. It is known that dilute solutions of nitric acid when in contact with apparatus constructed of ferrous metals exert a deleterious corrosive effect thereupon. As a consequence of this corrosive action, recourse had been had to the use of noble metals, highly expensive alloys, or to equally expensive lined equipment that are not appreciably at? tacked. This corrosion situation has resulted in placing an uneconomical cost disadvantage upon handling of such materials.

It is therefore an object of the present invention to provide a method of reducing the rate of corrosion of ferrous metals by a dilute aqueous solution of nitric acid to an extremely low rate.

It is a further object of this invention to provide dilute aqueous nitric acid solutions of reduced corrosiveness with respect to ferrous metals.

Another object of this invention is to provide a method for rendering dilute aqueous nitric acid solutions which normally corrode ferrous metals rapidly, much less corrosive in their action upon said ferrous metals.

Further and additional objects of this invention will be readily apparent from the disclosure and discussion herein below.

It has been disclosed in the prior art that ammoniacal solutions, such as ammoniacal ammonium nitrate, which are corrosive to ferrous metals with which they may come in contact, may be rendered less corrosive by the addition thereto of small amounts of such materials as thiocyanates and thiourea, sodium arsenite and others. It appears therefore that the materials heretofore employed are effective for inhibiting the corrosiveness of basic solutions toward ferrous metals.

We have now surprisingly found that dilute aqueous solutions of nitric acid which are much more highly corrosive to ferrous metals than the aforementioned basic solutions, can be effectively inhibited in their corrosive action toward ferrous metals by adding selected sulfurcontaining compounds to said acid solutions. We have further surprisingly found that alkali metal and ammonium arsenites in combination with the selected sulfurcontaining compounds used in the present invention provide dilute aqueous nitric acid solutions of still further reduced corrosiveness. This is especially surprising in view of the fact that we have found, in the case of sodium arsenite, that sodium arsenite alone provides no reduction in the corrosiveness of dilute aqueous nitric acid solution toward ferrous metals.

By the term ferrous metals, we mean to include within'the scope of this invention high carbon and low CROSS REFERENCE EXAM 3N ER Patented Nov. 6, 1956 carbon steels generally, boiler plate, cast iron, malleable cast iron, wrought iron, puddled wrought iron, austenitic steel, and copper-, molybdenumand nickel-containing steels or combinations thereof. In fact, all ferrous metals; which are corrodible by dilute nitric acid solutions are i included within the scope of this invention.

In accordance with this invention the corrosion of f ferrous metals by a dilute aqueous solution of nitric acid can be substantially reduced by adding to said nitric acid solution a compound or mixture of compounds from the class represented by the general formula wherein R is a radical selected from the group consisting s II o of hydrogen and alkyl groups containing not to exceed four carbon atoms, and wherein R1 is a radical selected from the group consisting of hydrogen, alkyl groups containing not to exceed four carbon atoms, and a thioamide radical, and wherein R and R1 together form heterocyclic ring structures containing up to five members consisting Y of carbon and nitrogen, said compounds containing not more than seven carbon atoms per molecule. Typical 4 compounds applicable to use in the present invention include: thiourea; l,3-dimethyl-2-thiourea; thiourea; l-methyl-3-isopropyl-2-thiourea; tert-butyl-Z-thiourea; 1-ethyl-3-tert-butyl-2-thiourea; lethybZ-thiourea; l-methyl-Z-thiourea; ethylidene thiourea dithiobiuret; 1-propyl2,4-dithiobiuret; thiobiuret', l-tert-butyl-2,4-dithiobiuret and the like. Thio- 1,3-diethyl-2- 1-methyl-3- having the following structural formula:

5 HN-i'J-NH H-C-CH:

ethylene thiourea; N,N-(tetra methyl-ethylene)-thiourea; Z,4-ditl1iobiuret; l-methyl-2,4-dithiobiuret; l-ethyl-2,4-

urea and dithiobiuret are available commercially. The i substituted thioureas may be made in any of the following ways: (1) by heating the thiocyanate of primary or secondary amines; (2) by adding hydrogen sulfide to a di-substituted cyanamide; (3) by adding ammonia or primary or secondary amines to mustard oil (isothiocyanates) RNCS; (4) by heating primary amines with carbon disulfide; and (5) by heating thiophosgene witharnines.

The arsenite compounds employed in our invention are those selected of the group consisting of sodium arsenite,

potassium arsenite, lithium arsenite, ammonium arsenite.

The quantity of our inhibitors which can be incorporated to reduce the rate of corrosion may vary from a very small percentage upward, depending upon the degree of corrosiveness of the nitric acid solutions to be v tions of nitric acid containing preferably not to exceed 15 weight percent of nitric acid at temperatures below about 3 150 F., more preferably at temperatures below about I F. At these low temperatures very small amounts of our inhibitors eflectively inhibit the corrosiveness of the acid to ferrous metals.

The following examples are intended to be illustrative I The total weight percent of inhibitor, j

of underlying principles and should not be construed as unduly limiting our invention.

EXAMPLE I .The results of the following tests demonstrate that the compounds disclosed in the present invention substantially reduce the corrosiveness of aqueous solutions of nitric acid on ferrous metals. The test procedure involved following the weight loss of metal strips submerged in inhibited and uninhibited or control test solutions in tightly stoppered, wide mouthed, four-ounce bottles. The test solution was 15 weight percent aqueous nitric acid. v

Plates of low carbon steel were used as test specimens. The plates were cut from a sheet of steel and were approximately wide and 3" long having a total area of 2.67 square inches exposed to the corrosive solution. A coating of mill scale was removed from each specimen by a wire bufiing wheel before the test and the plates were clean and shiny.

The bottles were filled with 15 weight per cent aqueous nitric acid, approximately 120 cc. weighed quantities of the inhibitor being tested were added and the bottles thoroughly shaken until the inhibitors were completely dissolved. The test specimens were pickled for five minutes in 36 weight percent hydrochloric acid before the tests were started. The weighed plates were placed in the solutions where they remained for two hours under static conditions at a temperature of 75 F. The plates were then removed, washed with water, dried, and weighed to determine the extent of corrosion as a function of time.

The results expressed in terms of corrosion in inches per day, are tabulated below.

Corrosion of low carbon steel at 75 F. caused by 15% nitric acid (Corrosion rates expressed in inches per day) The test procedure of Example I was repeated using inhibitors of the present invention at a temperature of 100 F. The results, expressed in terms of corrosion in inches per day, are tabulated below:

Corrosion of low carbon steel at 100 F. caused by 15% nitric acid (Corrosion ratio expressed in inches per day) Percent Percent Corrosion, Reduction No. Inhibitor Inhibitor In./Day, in

Added -2 Hrs Corrosion ate 0 0.192 0 0. 198 0. 10 0. 0089 95. 44 d0.. 0.10 0.0090 95.38 Ethylene Thlouretu- 0. 10 0. 0079 95. 95

EXAMPLE III In addition to the tests reported in the previous examples a qualitative test was made wherein a 0.10 weight percent solution of dithiobiuret in weight percent nitric acid was employed as test solution. The appearance of the metal strips was observed for evidence of corrosion. At a concentration of 0.10 percent for 4 dithiobiuret, good protection was provided over a period of two hours.

EXAMPLE IV In addition to the tests described in the previous examples wherein sulfur-containing compounds of the present invention were used alone, tests were also conducted wherein they were used in combination with sodium arsenite. The test procedure of Example I was employed and the tests were conducted at 75 F. The results, expressed in terms of corrosion in inches per day, are recorded in the following table.

TABLE 1 Corrosion of law carbon steel at 75 F. caused by 15% nitric acid Percent Percent Corrosion, Reduction N 0. Inhibitor Inhibitor In./Day, in

Added 0-2 Hrs. Corrosion Rate 0 0. 192 0 0. 198 0. 10 0. 198 None 0. 10 0.0025 98. 72 0.10 0.0038 98.05 3' {8 0. 0003 99.85 0. 10 {Sodium arsenite..-- 0. 10 0016 18 The above results show that sodium arsenite alone provides no reduction in corrosion when compared to the control. However, in combination with thiourea the reduction is even more pronounced than that provided by thiourea alone.

While this invention has been described and exemplified in terms of its preferred embodiments those skilled in the art will readily appreciate that modification can be made without departing from the spirit or scope of the invention.

We claim:

1. A process for reducing the corrosion of ferrous metals by a dilute aqueous nitric acid solution, which comprises incorporating in said acid solution both a compound represented by the general formula wherein R in a radical selected from the group consisting of hydrogen and alkyl groups containing not to exceed four carbon atoms, and wherein R1 is a radical selected from the group consisting of hydrogen, alkyl groups containing not to exceed four carbon atoms, and a thioamide radical, and wherein R and R1 together form heterocylic ring structures containing up to five members consisting of carbon and nitrogen, said compound containing not more than seven carbon atoms per molecule, and an arsenite selected from the group consisting of sodium arsenite, potassium arsenite, lithium arsenite and ammonium arsenite, in amounts sufiicient to decrease the rate of corrosion of a ferrous metal by said acid solution.

2. A process for reducing the corrosion of ferrous metals by an aqueou acid solution containing about 15 weight percent nitric acid, which comprises incorporating into the said acid solution from 0.05 to 2.0 weight percent of a compound represented by the general formula wherein R is a radical selected from the group consisting of hydrogen and alkyl groups containing not to exceed four carbon atoms, and wherein R1 is a radical selected from the group consisting of hydrogen, alkyl groups containing not to exceed four carbon atoms, and a thioamide radical, and wherein R and R1 together form heterocylic ring structures containing up to five members consisting of carbon and nitrogen, said compound containing not more than seven carbon atoms per molecule, and from to 1.95 weight percent of an arsenite selected from the group consisting of sodium arsenite, potassium arsenite, lithium arsenite and ammonium arsenite, the total weight percent of inhibitor not exceeding 2.0 percent.

3. A process for reducing the corrosion of ferrous metals by an aqueous acid solution containing about 15 weight percent nitric acid, which comprises incorporating in said acid solution from 0.05 to 2.0 weight percent of a compound selected from the group consisting of thiourea, ethylene thiourea, and dithiobiuret, and from 0 to 1.95 weight per cent of sodium arsenite, the total weight percent of inhibitor not exceeding 2.0 percent.

4. A process according to claim 3 in which 0.05 to 1 percent thiourea and 0.05 to 1 percent sodium arsenite are added to the aqueous acid solution.

5. A process according to claim 3 wherein ethylene thiourea is added to said aqueous acid solution.

6. A process for reducing the corrosion of a lowcarbon steel by a dilute aqueous nitric acid solution, which comprises incorporating in such acid solution thiourea and sodium arsenite in amounts suflicient to decrease the rate of corrosion of said low-carbon steel by said acid solution.

7. A process for reducing the corrosion of a low-carbon steel by an aqueous acid solution containing about 15 weight percent nitric acid at temperatures up to 200 R, which comprises incorporating in said acid solution from 0.05 to 2.0 weight percent thiourea and from 0 to 1.95 weight percent sodium arsenite, the total weight percent of inhibitor not exceeding 2.0 percent.

8. As a new composition of matter of reduced corrosiveness with respect to ferrous metals, dilute aqueous nitric acid solution contain g a compound represented by the general formula I H s H N EL N R/ Ri wherein R is a radical selected from the group consisting of hydrogen and alkyl groups containing not to exceed four carbon atoms, and wherein R1 is a radical selected from the group consisting of hydrogen, alkyl' groups containing not to exceed four carbon atoms, and a thioamide radical, and wherein R and R1 together form' heterocyclic ring structures containing up to five mem bers consisting of carbon and nitrogen, said compound containing not more than seven carbon atoms per mole-1 cule, and an arsenite selected from the group consisting;

of sodium arsenite, potassium arsenite, lithium arseniteand ammonium arsenite, in amounts suflicient to decrease the rate of corrosion of a ferrous metal by said?v solution.

9. As a new composition of matter, a dilute aqueous acid solution containing about 15 weight percent nitric acid and thiourea and sodium arsenite in amounts sufiicient to decrease the rate of corrosion of a ferrous metal by said solution.

10. As a new composition of matter, a dilute aqueous nitric acid solution containing ethylene thiourea and sodium arsenite in amounts suflicient to decrease the rate of corrosion of a ferrous metal by said solution.

References Cited in the file of this patent UNITED STATES PATENTS Schmidt Nov. 30, 1926 

1. A PROCESS FOR REDUCING THE CORROSION OF FERROUS METALS BY A DILUTE AQUEOUS DILUTE NITRIC ACID SOLUTION, WHICH COMPRISES INCORPORATING IN SAID ACID SOLUTION BOTH A COMPOUND REPRESENTED BY THE GENERAL FORMULA
 9. AS A NEW COMPOSITION OF MATTER, A DILUTE AQUEOUS ACID SOLUTION CONTAINING ABOUT 15 WEIGHT PERCENT NITRIC ACID AND THIOUREA AND SODIUM ARSENITE IN AMOUNTS SUFFICIENT TO DECREASE THE RATE OF CORROSION OF A FERROUS METAL BY SAID SOLUTION. 